In filters for separating solids from liquids, particularly nutsch filters, it is known to provide a stirring apparatus above the filter plate which can be used for different purposes. The stirring apparatus essentially comprises a rotary driving shaft which can be raised and lowered. Two or more stirring apparatus arms are secured to the shaft. As a function of the rotation direction and/or the raising or lowering movement, it is possible to perform additional operations.
An important operation to be performed by the stirring apparatus consists of leveling or smoothing the filter cake forming on the filter plate. During the filtering operation, it is unavoidable that the filter cake will build up in a non-uniform manner and cracks can also form. Therefore, the stirring apparatus has the function of leveling or smoothing the filter cake, so that the cracks therein close, in order to ensure a uniform permeability of the filter cake.
For the purpose of leveling the filter cake, it is necessary to exert a given pressure thereon. In addition, this pressure is to be exerted with the same intensity independently of the expansion or shrinkage of the filter cake. In practical operations, particularly when a nutsch filter is used, it is difficult to satisfy the aforementioned requirements. Therefore the nutsch filter operator manually readjusts the smoothing of the filter cake in such a way that despite expansion or shrinkage of the filter cake, the pressure of the stirring apparatus remains approximately constant. However, this manual adjustment of the stirring apparatus is time-consuming, so that the operator is unable to perform other functions having to permanently observe the filtration operations. This can be made more difficult by poor observation possibilities, e.g., due to a fogged or dirty inspection glass.